On the market, voltage-source converters are available for three-phase drives up to several MW whose self-commutated power converter has a plurality of high-power modules. When working with these amounts of power, the self-commutated power converter has two inverters which are electrically connected in parallel. FIG. 1 shows an equivalent circuit diagram of a self-commutated power converter 2 of that kind. The two parallel-connected inverters 4 and 6 have IGBT high-power modules T1 through T6 and T1' through T6', respectively, as converter valves. IGBT high-power modules T1 through T6 of inverter 4 are mounted on a heat sink 8 (FIG. 2). In the same way, IGBT high-power modules T1' through T6' of inverter 6 are mounted on a heat sink 10 (FIG. 2).
In FIG. 2, these heat sinks 8 and 10 are illustrated with the appertaining IGBT high-power modules T1 through T6 and T1' through T6'. It can be deduced from this representation that IGBT high-power modules T1 through T6 and T1' through T6', respectively, are arranged one over another in the direction of flow of the cooling air. The heat sink used is shown in e.g., FIG. 4 of European Patent 0 658 934.
A disadvantage of this design of the power section of inverters 4 and 6 is that at least uppermost modules T1 and T1' receive a heated cooling air. The cooling air flowing in it heat sinks 8 and 10, respectively, heats up from IGBT module to IGBT module. Consequently, the cooling air can dissipate significantly less power loss at uppermost modules T1 and T1' than the cooling air at lowest modules T6 and T6'. To prevent inverters 4 and 6 from failure due to overheating of uppermost modules T1 and T1', respectively, self-commutated power converter 2 is technically rated by uppermost modules T1 and T1', respectively. Because of this, a power reduction of 20% can occur for self-commutated power converter 2. This means that, possibly, two IGBT high-power modules electrically connected in series must be provided per converter valve for a self-commutated heavy-duty power converter. In this manner, the illustrated problem would further intensify when working with an arrangement shown in FIG. 2, as a result of which one would have to expect a further increase in the power reduction. This means that the efficiency would further decline.
Moreover, these additional IGBT modules influence the size, the weight and the cost of the self-commutated power converter and, consequently, of the voltage-impressing converter, as well. Furthermore, when connecting at least two modules in series within such a converter arm, slightest differences in the switching times can already lead to unsymmetrical voltage sharing. Voltage differences of that kind can be prevented by an additional measure in the control. This additional outlay can be prevented by dividing the supply capacitor voltage into two parts by a center tap so that, in addition to the two potentials plus and minus, a potential zero results. By connecting the zero potential via in each case two additional midpoint diodes of each pair of arms, a "three-point power converter" results which is self-commutated.
In a paper entitled "Transistorwechselrichter fur Nahverkehrstriebfahrzeuge [Transistor Inverters For Outer-Suburban Service Traction Vehicles]", published in German Periodical "eb - Elektrische Bahnen [Electric Railways]" No. 11, pages 341 through 347, such a three-point inverter is depicted as self-commutated power converter. A three-point inverter of that kind uses the same number of modules per inverter arm as a two-point inverter having two modules per converter valve, respectively, but has significant advantages, which are discussed in chapter 3.1 of this paper.
Commercial IGBT modules are supplied as encapsulated modules which contain the actual IGBT and an antiparallel free-wheeling diode. Because of the isolated design of the semiconductors inside the module, the IGBT modules of the three-point inverter can be mounted on a shared heat sink without further measures for electrical isolation. The cooling air brushes over the lower side of the heat sink and does not come into contact with the electric components on the upper side. In this arrangement, only three IGBT modules are arranged side by side along the direction of flow of the cooling air. This arrangement has the disadvantage that, in the case of a fault of an inverter phase, the outlay for replacing one or more modules is quite considerable.